Oxysomes showing phosphorylation
Plant Physiology

Phosphorylation of ADP- Chemosynthetic hypothesis.

The process of the formation of the chemical energy (ATP) by using the concentration gradient of the H+ ion is called the Phosphorylation.

Note– Phosphorylation is just the addition of phosphate to any compound. But in this post we read only phosphorylation of ADP to form ATP.

And the theory given to explain ATP formation is the Chemosynthetic hypothesis.

The chemosynthetic hypothesis was given by Peter Mitchell.

The structure essential for the chemosynthesis is the Oxysome, also known as the ATP synthetase.

Oxysome is a tennis racket like structure consists of the 2 parts-

  1. F0– It is embedded inside the cell membrane. It provide the passage for the H+ ion to cross.
  2. F1– It is the part which have the enzymatic activity and convert ADP(Adenosine diphosphate) and iP(inorganic phosphate) into the ATP(Adenosine triphosphate).
Oxysomes showing phosphorylation
Oxysomes (ATP synthetase) showing phosphorylation.

Chemosynthetic hypothesis.

According to the chemosynthetic hypothesis the crossing of H+ ions through the F0 part causes the structural changes in the F1, which activate its enzyme activity thus forming the ATP.

One can say that the Kinetic Energy of the ions converts into the chemical energy or bond energy in the form of ATP. All thanks to the Law of Conservation of Energy.

Note – Two H+ ions has to pass through the oxysome to form one ATP.

On the basis of the formation, site and the cause of the gradient it can be differentiated into 2 types-

  1. Oxidative Phosphorylation
  2. Photo-phosphorylation

While the process of the ATP in both the process are same, but what is the cause that lead the gradient to be formed and where the gradient is formed? Make the process different.

Oxidative phoshorylation

Formation of the ATP by using the energy of oxidation (combustion) of the food in the presence of oxygen (O2) is called the Oxidative phosphorylation.

Site-

It occurs in the mitochondria, the inner membrane of the mitochondria contain oxysomes facing toward the matrix.

Mitochondria
Mitochondria showing only oxysomes. Img source

Why?

The series of Oxidative reactions occurs in the inner membrane of the mitochondria, for which different complexes are responsible, combinely lead into the formation of gradient.

Oxysome is also called as the Comlex V.

How?

All this complexes together forms the Electron transport chains, which takes the Hydrogen from the NADH2 and transfer only electron to the next complex.

Resulting in the release of H+ ion in the intermembrane space. Also some carrier of the chain take up some extra H+ ion from the matrix and transfer to the intermembrane space.

Regular increase in the concentration of the H+ ion in the intermembrane space and decrease in the concentration of the H+ ion in the matrix, results into creating a Hydrogen gradient.

Thus activating the oxysomes which further lead into the transfer of H+ ion from intermembrane space leading into the formation of ATP from ADP and iP by the F1 part of oxysome.

Note– 1 NADH2 molecule forms the 3ATP molecules. And 1 FADH2 molecule form the 2 ATP molecules.

Photo-phosphorylation

Using the light energy to form the form the ATP is photophosphorylation.

Site-

It occurs in the chloroplast during the light reaction of the photosynthesis.

More specific – it happens in the lumen of the thylakoid.

Why?

Due to the light reaction, the mechanism of the light reaction structured in such a way that it lead in the formation of Hydrogen gradient between thylakoid lumen and stroma.

How?

Photolysis of the water inside the lumen of thylakoid result in the release in the Hydrogen ion (H+), electron (e-) and Oxygen.

The electron is used up by the chlorophyll and oxygen is evolved leaving the H+ behind.

Further the conversion of the NADP into the NADPH2, result in the decrease of the H+ ions in the stroma.

This end up making the Hydrogen gradient and the process of chemosynthesis is repeated and we get the newly formed ATP molecules.

Two type of photo-phosphorylation is found in plants

Non-cyclic Photo-phosphorylation-

  • The extruded electron never returns to the same photosystem.
  • Continous supply of electron is required (remember photolysis of water). Oxygen is also evolved.
  • Both photosystem takes part in the process and occurs on the granum thylakoid.

Cyclic Photo-phosphorylation-

  • Extruded electron return back to the same photosystem.
  • No need of photolysis of water and oxygen doesn’t evolve.
  • Occurs on the stroma lamellae and only one photosystem take part in it (PS-I).

To known more about Chloroplast structure click here.

Important points

  • Essential requirement of the chemosynthesis are the membrane, a gradient and the ATP synthatase (oxysome).
  • Oxidative phosphorylation occurs in both plants and animals, but photo-phosphorylation occurs only in plants.
  • F0 part of the oxysome is an example of the intrinsic protein of the membrane.
  • Whether the basic process of the phosphorylation is same in both type but the mechanism i.e. the formation of gradient is entirely different.

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